In a cooling tower used at a plant facility or the like, heat exchange is performed between cooling water and high-temperature exhaust gas discharged from a boiler or the like. A portion of the cooling water becomes steam due to this heat exchange, so the ions or silica (SiO2) in the cooling water are concentrated. Accordingly, the cooling water discharged from the cooling tower (blowdown water) reaches a state with a high ion concentration or silica concentration.
Water containing large quantities of ions is released into the environment after being desalinated. Known examples of apparatuses for performing desalination treatment include a reverse osmosis membrane apparatus, a nanofiltration membrane apparatus, and an ion exchange membrane apparatus.
Of the ions contained in the water described above, monovalent positive ions such as Na+, K+, and NH4+ or negative ions such as Cl− and NO3− are ions with high solubility in water. On the other hand, divalent metal ions such as Ca2+, negative ions such as SO42− and CO32−, or silica are components constituting scale. Since salts or silica serving as components constituting scale have low solubility with respect to water, they are easily deposited as scale. In particular, Ca2+, SO42−, and carbonic acid ions (CO32− and HCO3−), and silica are contained in abundance in brine, industrial waste water, and blowdown water of cooling towers. When scale is generated inside the apparatus for performing the desalination treatment described above, the treatment capacity is diminished. Therefore, there is a demand to perform desalination treatment without generating scale.
The lime-soda method is known as a method for removing Ca2+. In the lime-soda method, sodium carbonate is added to water to be treated, and the Ca2+ in the water to be treated is deposited/precipitated as calcium carbonate and thereby removed from the water.
Patent Document 1 discloses a waste water treatment system using the lime-soda method in which a chemical softening apparatus, an ion exchange apparatus, a reverse osmosis membrane apparatus, and the like are combined.
In addition, a method for removing silica from large quantities of waste water is disclosed in Patent Document 2, and it is proposed to add a sodium aluminate solution to waste water so as to substantially reduce the silica concentration in the waste water.